As a method of mounting electronic components on circuit boards and the like, a method which uses solder bumps formed on the substrate electrodes is known. Further, when mounting a substrate with solder bumps to another substrate, the bumps of the substrate with solder bumps are mounted in positions opposing the electrodes of the other substrate that is to be connected, and by subsequently heating at a temperature of melting the solder bumps (reflow) or higher, the electrodes on the two substrates can be soldered together. In semiconductor packages, demands for smaller and thinner electronic equipment, increased functionality and higher performance have lead to an increase in the number of I/O electrodes (terminals), and a shortening of the distance between electrodes (terminals) (a narrower pitch). As a result, the configuration of semiconductor packages has changed from configurations using lead connections, such as QFP (Quad Flat Package), to configurations using flip-chip connections, which is a connection method in which bumps are positioned on the bottom of chips and used for connection purposes, such as BGA (Ball Grid Array) and CSP (Chip Scale Package). In the flip-chip connection method, solder bumps are formed in advance on the electrodes on a circuit board, and these solder bumps are joined to the circuit electrodes of a substrate or the like. Known methods for forming solder bumps include a method in which solder balls are mounted on the circuit electrodes of electronic components and the like, and a method in which a cream solder or a solder paste composed of a liquid or paste containing solder particles is printed onto the electrodes of a circuit board through the through-holes in a metal mask.
In the solder ball mounting method, because solder balls that have been prepared in advance are mounted, by altering the size of the prepared solder balls and narrowing the particle size distribution, the necessary bump height can be achieved, and fluctuation in the bump height can be suppressed to low levels. However, preparation of solder balls that satisfy these standards is costly. Further, the process of mounting the solder balls on the electrodes is complex, and introducing a solder ball mounting apparatus requires considerable capital investment. On the other hand, in the solder paste (cream solder) printing method, because air is incorporated within the paste during the printing operation, voids tend to be generated within the bumps. Further, there are fluctuations in the bump height, and opposing electrodes can sometimes not be joined.
In recent years, simple processes that utilize the self-assembly of a solder component, either on electrodes or between opposing electrodes, have been proposed as novel methods for forming solder bumps and novel methods for joining opposing electrodes. For example, in Patent Document 1, solder bumps are formed on electrodes using a paste comprising a solder powder, a convection additive and an epoxy resin. The paste is supplied to the electrode surface side of a substrate comprising electrodes, and by subsequently placing a flat plate on the paste and heating at a temperature of melting the solder powder, or higher, and at a temperature of boiling the convection additive or higher, gas bubbles of the convection additive generate a convection flow. This stirs the melted solder powder, causing bonding of particles of the solder powder, and the solder powder grows to a uniform size and accumulates on the electrodes. As a result, bumps having high uniformity are formed. In a similar manner, Patent Document 2 discloses that the opposing electrodes can be joined by using a solder. Further, after the opposing electrodes are joined by using the solder, an underfill can be formed by curing the epoxy resin scattered across the regions outside of the opposing electrodes.
The paste disclosed in the aforementioned Patent Document 1 provides a simpler process than the solder paste printing method or the solder ball mounting method, but because the process uses a liquid or paste-like composition, there are some problems such as inferior storage properties, transportability and handling properties during use. Further, although fluctuations in the height of the solder bumps can be suppressed, because gas bubbles are generated by the convection additive, these gas bubbles are readily incorporated within the bumps, and as a result, the void is prone to be formed in the bumps. Furthermore, in those cases when the curing of the thermosetting resin proceeds before the solder component accumulates on the electrodes, completion of the self-assembly of the solder component on the electrodes becomes difficult. Moreover, in the resin washing step performed after the heating, because of curing of the thermosetting resin, the resin cannot dissolve, or cannot be completely removed by washing.
Further, when opposing electrodes are joined by using solder in the manner described in Patent Document 2, a small amount of solder residue which has not migrated between the opposing electrodes is retained within the epoxy resin scattered across the regions outside of the opposing electrodes. This solder residue reduces the electrical resistance between adjacent electrodes, and can therefore cause short-circuits.